The present invention relates to ceramic foam, especially ceramic foam filters and a process for manufacturing the same.
Porous ceramic foam materials are known in the art. These porous ceramic foam materials are known to be particularly useful in filtering molten metal, especially molten aluminum and aluminum alloys. U.S. Pat. Nos. 3,893,917, 3,947,363, 3,962,081, 4,024,056, 4,024,212, 4,075,303, 4,265,659, 4,342,644 and 4,343,704 describe some of the ceramic foam materials which have been used as filters.
Ceramic foam filters are typically produced by impregnating an open cell, hydrophobic flexible foam material with an aqueous slurry containing a ceramic material, squeezing the organic foam to remove excess slurry, using an air knife to open up any surface blockage, drying the impregnated foam and heating it to burn off the organic components, and firing the body at an elevated temperature to produce the filter. A wide variety of slurries have been used in an attempt to form a ceramic foam filter having considerable high temperature resistance and a structure resistant to degradation under the severe conditions of use associated with molten metal filtration. In U.S. Pat. No. 4,610,832, a molten metal filter is prepared using an aqueous slurry comprising a thixotropic ceramic composition including a gelled alumina hydrate. Co-pending U.S. Patent Application Ser. No. 125,943, filed Nov. 27, 1987, now abandoned, describes a ceramic foam filter prepared from a ceramic slurry containing a phosphate binder. Co-pending U.S. patent application Ser. No. 29,184, filed Mar. 23, 1987, now U.S. Pat. No. 4,885,263, describes a ceramic foam filter prepared from a ceramic slurry containing silicon carbide and a colloidal silica binder.
In some manufacturing processes, the ceramic slurry contains ceramic fibers. U.S. Pat. No. 4,265,659 illustrates one such process wherein ceramic fibers are added to a slurry containing alumina, chromia, bentonite, kaolin and an air settling agent. U.S. Pat. No. 4,391,918 also illustrates a process wherein a ceramic fiber material is added to the slurry. The fibers are intended to act as a crack growth inhibitor and thereby increase the strength of the filter element.
Flocked polyurethane foams are currently used for humidifiers and the like where the flocking acts to increase surface area. These types of foams are generally ill suited to the manufacture of ceramic foam filters because the extensive amount of flocking impedes slurry penetration.
Filters manufactured using polyurethane reticulated organic foams have encountered certain problems when the process involves certain types of slurries. Filters produced without supplemental processes such as spraying or dipping after impregnation can have weak or friable surfaces, poor edge quality or, in some cases, can be intrinsically weaker than desired for some applications. Processes including spraying or dipping after impregnation have been used in an attempt to improve the ceramic coating. Often, surfaces of the foam substrate have sharp projections which are difficult to coat. Moreover, the webs of the foam substrate are generally triangularly shaped and the peaks of the triangle especially may not be well covered. Spraying after impregnation however often creates an undesirable laminar effect. The sprayed layer is applied to a dried layer and the bonding is weak. Also, spraying can wash away some of the initial dried, impregnated slurry and weaken the structure. Spraying must, therefore, be carefully controlled adding to manufacturing complexity.
Accordingly, it is an object of the present invention to provide a ceramic foam having an improved ceramic coating.
It is a further object of the present invention to provide a ceramic foam filter as above wherein the slurry effectively coats the filter web.
It is still a further object of the present invention to provide a ceramic foam filter having improved mechanical properties.
It is yet a further object of the present invention to provide a process for forming the above ceramic foam filter.
These and other objects and advantages will become more apparent from the following description.